Lecture20 - Chapter 10: Phase Transformations ISSUES TO...

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Chapter 10 - 1 ISSUES TO ADDRESS. .. Transforming one phase into another takes time. How does the rate of transformation depend on time and T ? How can we slow down the transformation so that we can engineer non-equilibrium structures? Are the mechanical properties of non-equilibrium structures better? Fe γ (Austenite) Eutectoid transformation C FCC Fe 3 C (cementite) α (ferrite) + (BCC) Chapter 10: Phase Transformations
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Chapter 10 - 2 Phase Transformations Phase Transformation – a new phase is formed within the parent phase Nucleation nuclei (seeds) act as template to grow crystals for nucleus to form rate of addition of atoms to nucleus must be faster than rate of loss Growth once nucleated, grow until reach equilibrium
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Chapter 10 - 3 r * = critical nucleus : nuclei < r * shrink; nuclei> r * grow (to reduce energy) Adapted from Fig.10.2(b), Callister 7e. Homogeneous Nucleation & Energy Effects G T = Total Free Energy = G S + G V Surface Free Energy- destabilizes the nuclei (it takes energy to make an interface) γ π = 2 4 r G S γ = surface tension Volume (Bulk) Free Energy stabilizes the nuclei (releases energy) υ π = G r G V 3 3 4 volume unit energy free volume = υ G
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Chapter 10 - 4 Solidification T H T r S m γ - = 2 * Note:   H S = relative insensitive to T changes γ =   weak function of   T r * decreases as   T increases, i.e., as T decreases For typical   T r * ca. 100Å H S = latent heat of solidification = Δ H f   T m = melting temperature γ = surface free energy T = T m - T = supercooling r* = critical radius
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Chapter 10 - 5 Superheating/Supercooling Upon crossing a phase boundary on the composition temperature phase diagram phase transformation towards equilibrium state is induced. But the transition to the equilibrium structure takes time and transformation is delayed. During cooling, transformations occur at temperatures less than predicted by phase diagram: supercooling . During heating, transformations occur at temperatures greater than predicted by phase diagram: superheating. Degree of supercooling/superheating increases with rate of cooling/heating. Metastable states can be formed as a result of fast temperature change. Microstructure is strongly affected by the rate of cooling.
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Chapter 10 - 6 Driving force to nucleate increases as we increase T supercooling (eutectic, eutectoid) superheating (peritectic) Small supercooling few nuclei - large crystals Large supercooling rapid nucleation - many nuclei, small crystals
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Chapter 10 - 7 Solidification: Nucleation Processes Homogeneous nucleation nuclei form in the bulk of liquid metal requires supercooling (typically 80-300°C max) Heterogeneous nucleation much easier since stable “nucleus” is already present Could be wall of mold or impurities in the liquid phase allows solidification with only 0.1-10ºC supercooling
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Chapter 10 - 8 Rate of Phase Transformations Kinetics - measure approach to equilibrium vs.
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Lecture20 - Chapter 10: Phase Transformations ISSUES TO...

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